Conventional multi-stage centrifugal pumps provide a plurality of impellers on the same shaft, and produce a high pressure head. Sealing, supporting and guiding the shaft (with bearings), and the balancing of axial forces at the non-driven end of such pumps, are typically complicated procedures. For example in some conventional constructions the non-driven end of the shaft is mounted by a pair of tapered roller bearings which are spaced a significant distance from the pump itself since sealing of the pump is carried out by a conventional packing (which is relatively long). Since the packing must be occasionally replaced, it must be possible to remove the packing. A balancing drum is preferably also provided between the packing and the last impeller on the shaft, for balancing a majority of the axial forces generated by the pump. Centrifugal pumps always generate an axial force causing the impellers to move toward the suction channel. The balancing drum typically has a labyrinth seal, and between the cylindrical surfaces of the balancing drum there is a gap of about 0.05 mm. The pressure generated by the pump is introduced into the cavity between the last impeller and the balancing drum so that the pressure of the pump against the balancing drum tends to push the balancing drum further away from the impeller. The force that is thus generated is counter-directional to the axial force generated by pumping so that the axial force loading the bearings of the pump is the difference between the axial forces having different directions.
In most conventional multi-stage centrifugal pumps the length of the actual pump (shaft and impellers) is only about 55% of the total length of the apparatus. A substantial portion of the length of the entire assembly is due to the spaced location of the bearings at the non-driven end as a result of the packing construction. This requires a sturdier and longer shaft than is desired, the sturdier construction being necessary to resist the bending load on the shaft because of its length.
According to the present invention about 20% (e.g. about 17-28%) of the length of a conventional multi-stage centrifugal pump assembly is saved by providing the balancing, bearing, and sealing functions in a simplified manner.
According to one aspect of the present invention a multi-stage centrifugal pump is provided comprising the following elements: A rotatable elongated shaft having first and second ends. A casing including a stationary end structure, an inlet for fluid to be pumped, and an outlet for pumped fluid. A plurality of impellers mounted on the shaft within the casing, for rotation with the shaft. And, adjacent at least one end of the shaft within the casing first and second annular slide bearings, the first slide bearing mounted to the stationary end structure, and the second slide bearing mounted to the shaft for rotation therewith. The second slide bearing may be mounted to the shaft for rotation therewith by one or more flexible mounting devices, such as a plurality of O-rings. A sleeve may also be mounted to the shaft for rotation with it, the sleeve provided between the O-rings and the shaft. First and second shoulders may be connected to the sleeve and engage opposite ends of the second slide bearing for positively positioning it within the end structure, and the second shoulder, on the opposite side of the second slide so bearing from the impellers, may have a labyrinth seal.
Typically there is a clearance between the first and second slide bearings of about 0.04-0.1 mm (preferably about 0.05 mm), and a lubricant (such as the pumped fluid) is provided in the clearance. The slide bearings may be made of silicon carbide, antimony carbide, carbon impregnated polytetrafiuoroethylene, or a similar material which performs a bearing function over a long period of time without degradation, and has good lubrication properties. The lubricant typically forms a liquid film on the bearing surfaces, which provides good lubrication.
According to another aspect of the present invention a multi-stage centrifugal pump is provided comprising the following components: A rotatable elongated shaft having first and second ends. A casing including a stationary end structure, an inlet for fluid to be pumped, and an outlet for pumped fluid. A plurality of impellers mounted on the shaft within the casing, for rotation with the shaft. And, means for simultaneously balancing axial forces in the casing, sealing the pump at the end structure, and supporting and guiding the shaft during rotation in the casing, the means comprising first and second annular slide bearings. The first and second annular slide bearings and associated components may be constructed as described above.
Another problem associated with conventional multi-stage centrifugal pumps is that each impeller is mounted to the shaft by way of a separate key in a keyway. The keyways are typically circumferentially equally spaced apart from each other, e.g. within about 120.degree. from each other, in the shaft. The manufacture of multi keyways is expensive and time consuming because the rounded ends of the keyways must be machined and that is a relatively complicated operation. Also shaft deformation can occur in a keyway, and other disadvantageous results may occur.
According to another aspect of the present invention problems associated with keying each of the impellers of a centrifugal pump to the shaft are avoided by providing a connection between at least some of the impellers to each other which takes the place of keying. For example at least some of the impellers may be mounted to the shaft by polygonal protrusions concentric with a shaft cooperating with similarly shaped polygonal recesses in an adjacent impeller. Therefore only one or two of the impellers need be keyed or otherwise attached to the shaft, and no keyways are necessary for the impellers connected to each other; or one or more sleeves cooperating with the impellers may be keyed or otherwise attached to the shaft so that no keyways for impellers are necessary at all.
Thus according to another aspect of the present invention a multi-stage centrifugal pump is provided comprising: A rotatable elongated shaft having first and second ends. A casing including an end structure, an inlet for fluid to be pumped, and an outlet for pumped fluid. A plurality of impellers mounted on the shaft within the casing, for rotation with the shaft. And, at least some of the impellers being mounted to the shaft by polygonal protrusions concentric with the shaft cooperating with similarly shaped polygonal recesses in an adjacent impeller, the impellers themselves devoid of key connections directly to the shaft.
It is the primary object of the present invention to provide an improved multi-stage centrifugal pump. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.